Main Sequence Stars
Generally stars spend most of their active lives travelling through the stage of stellar life-cycle known as the
Main Sequence. I say 'active' life because the remnants of dead stars can persist more or less indefinitely,
slowly fading into obscurity. I say 'generally' because even dead stars sometimes find ways to obtain new life
(see stellar accretion for example).

Main sequence stars fall into several spectral classes, as shown above. Going from top left to bottom right we
have, a small dim red dwarf (spectral class M), an orange dwarf (spectral class K) a yellow dwarf, like the Sun
(spectral class G), white stars (spectral class F on the left and the larger class A on the right) and the large hot
and blue B class stars, bottom left, and O class stars, bottom right.

The colours of these stars depend upon the surface temperature, with red being the coolest, followed by
orange, then yellow, then white and finally blue. In fact the blue B and especially the O class stars are so hot
that much of their energy is emitted as ultraviolet radiation that is invisible to human eyes.

The temperature, and hence colour of a star, is dependent largely on the star's mass. The table below
illustrates the masses, radii and luminosities of each main sequence star class; mass, radius and luminosity are
given relative to that of the Sun (1), so a B class star is some 500 000 times more luminous than the Sun,
temperature is given in degrees K (to convert to degrees C subtract 273, which makes a negligible difference
here), MS lifespan is the time spent on the main sequence:

Spectral Class        Colour        Mass        Radius        Luminosity        Temperature        MS Lifespan (yrs)

    M                       red              0.1          0.1               0.001                   3 000                100 billion
    K                     orange          0.5           0.3                0.03                    4 500                15 billion
    G                     yellow           1              1                   1                         5 500                10 billion
    F                      white             1.5          1.2                5.0                      7 000                 5 billion
    A                      white             2.5          2                   50                       9 000                 400 million
    B                      blue              10            5               10 000                  17 000                10 to 100 million
    O                      blue              40+        20              500 000                40 000                 2-8 million

So, more massive stars are larger, hotter and much more luminous. These figures are ball-park figures, since
each class overlaps with the adjacent classes on either side, such that there is a whole continuous spectrum of
star masses from about 0.08 to 150 times the Sun's mass. In fact the upper limit is not well established, but
once stars reach 30+ solar masses they become unstable and tend to rapidly lose their excess mass by
blasting it off into space. Stars smaller than about 0.08 solar masses are not hot enough to sustain nuclear
fusion and so shine very briefly, if at all.

Also dependent upon the mass of the star is the stars longevity (that is the length of time that it spends on the
Main Sequence). This can be estimated from the mass (M) and luminosity (L) using the approximation:

  MS lifetime = 10^10 x M/L yrs,

  (with M and L in units of solar mass and solar luminosity, both equal to 1 for the Sun)

A spectral class G star, which is a yellow dwarf and is the class to which the Sun belongs, burns for about 10
billion years (10 000 million years) before it runs out of fuel. The Sun is currently middle-aged and should burn
for another 5 billion years or so. Smaller stars, like the class M red dwarfs, may have less fuel to start with, but
they burn so dimly that their fuel lasts much longer and they have lifespans that exceed the current age of the
Universe. (The age of the Universe is established at 12.5 billion years and red dwarfs may burn for about 6
thousand billion years). At the other extreme, giant blue O and B class stars only burn for a few million or tens
of million years. Thus, wherever O and B class stars are found, new star formation has recently occurred. Such
regions include the bright blue spiral arms of the Milky Way Galaxy.

Most of the stars that you can see with the naked eye are the luminous white and blue stars. It is difficult to
ascertain the numbers of red dwarfs since they are so dim that they are hard to detect from great distances.

  A useful pneumonic: Oh Be A Fine Girl/Guy And Kiss Me

In addition, within each spectral class, there are ten
spectral subclasses, ranging from 0 to 9, with subclass 0
hotter than subclass 9. Thus we have classes like G2, the Sun (Sol) is a G2 star. The lower subclasses, toward
0, are also called
early spectral types, and those toward 9, later spectral types.

O stars and B stars, the UV-stars

These are the very hot O and B giant stars (blue giants) whose peek output is not in the visible spectrum at all,
but in the ultraviolet (UV). The heaviest O stars are
blue hypergiants. O stars are very hot and massive blue
giants that are so hot that helium becomes ionised in their outer atmosphere (helium, He, is a noble gas and
very stable with a high ionisation energy, meaning that it is very hard to remove electrons from helium atoms to
form positive helium ions) and they have strong spectral lines of singly ionised helium, He
II or He+. These
stars are very rare, and as of the year 2000, no O0, O1 or O2 and only a few O3 and O4 stars had been
discovered, with most known O stars being in the cooler O5 to O9 subclasses. However, more massive stars
are being discovered all the time. These stars have short lifespans of 3 to 5 million years or so, increasing if the
stars lose enough mass in their very intense stellar winds. O and B stars occur in the spiral arms of galaxies,
giving them their bright blue colour. In these regions, recent star formation has occurred, as these stars are
absent from older regions, having already left the main sequence during stellar death. O and B stars are often
found in small groups, called OB associations. Associations are loose groups of new stars, ranging from a few
to several hundred parsecs across, often found outside central
open clusters.

B stars have spectra dominated by the absorption lines of neutral helium (He
I) Be stars (e for emission) are
surrounded by shells of gas that, being heated by the central star, emit radiation as the atoms de-excite.

A Stars

These are massive and very luminous white giant stars and are also quite short-lived. Being young they are
often rotating rapidly and so appear stretched along their equator, like rugby balls (ellipsoid). Many A stars are
rotating close to the maximum rate, beyond which they would fly apart! A model of such a star is shown below:
Rapidly rotating A Star
Examples of A stars are Vega, Sirius, Deneb and Altair. The spectra of A stars are dominated by hydrogen
Balmer absorption lines (see
atomic spectra) which are strongest in A0 to A3 stars, with lines of ionised calcium
dominating in later spectral types. The effective surface temperature of these stars ranges from about 7500 to
9900 K.

AM Stars

these are stars of spectral type A0 to F0 that are slow rotators (and so more spherical) and have weaker
calcium lines, but stronger iron lines and very strong rare earths and heavier elements - a
heavy metal
. Most are short-period binary stars that co-orbit another star. The tidal forces between the two stars
act like a break, and this tidal breaking has slowed the rotation of these stars and increased the stability of
their atmospheres, such that heavier elements, like iron and the rare earths, have diffused up from the core
where they are generated by nuclear reactions. These high atmospheric concentrations of heavier elements
account for the anomolously strong spectral lines of these elements (spectral absorption lines are generated in
the star's upper atmosphere).

AP Stars

these are stars of spectral types B5 to F5 and have strong magnetic fields. They are also slow rotators, but
they are single rather than binary stars. They have increased concentrations of Mn (manganese), Si (silicon),
Eu (Europium), Cr (chromium) and Sr (strontium) in their atmospheres and with surface temperatures
decreasing in the same order from Mn-stars to Sr-stars.

F Stars

These stars are white to white-yellow with strong spectral absorption lines due to singly ionised calcium (Ca II).
Examples include: Canopus, Polaris, Procyon. The effective surface temperature of these stars ranges from
6000 to 7400 K.

G Stars

These are yellow stars (yellow dwarfs), with an effective surface temperature ranging from 4900 to 6000 K.
Spectral absorption lines of singly ionised calcium are dominant. (Ca+ or Ca
II has two prominent spectral
lines, one, the H line, occurs at 393.4 nm and the other, the K line, at 396.8 nm. Both these lines occur on the
edge of the visible (violet) / ultraviolet part of the spectrum). Most other metals are non-ionised at these
temperatures and give rise to neutral metal absorption lines. Some simple molecules are stable enough to form
in the atmosphere at these temperatures, and molecular bands (molecules have many spectral lines that
group into closely-spaced bands) of CH (carbon monohydride) and CN (carbon mononitride). Examples
include  Sol (the Sun) and Capella.

K Stars

These are orange dwarfs with effective surface temperatures in the range 3500 to 4900 K. They have strong
absorption lines of both Ca
II and neutral non-ionised calcium, Ca I. (At these cooler temperatures, not all the
calcium is ionised) and absorption lines due to other neutral metals are dominant and molecular bands are
stronger and increase in strength from hotter K0 stars to cooler K9 stars. Examples include: Arcturus and

M Stars

these are (relatively) cool red dwarfs. (Red giants and supergiants typically have spectral type M or S, but
these are not main sequence stars). The effective surface temperature ranges from 2400 to 3480 K. Oxygen is
in excess in their atmospheres, in the form of molecular oxides, such as Ti (titanium(II) oxide) and VO
(vanadium(II) oxide) that give rise to molecular absorption bands. Neutral metal absorption lines are also
strong, since temperatures are not high enough to ionise most metals.